The present invention relates to a coplanar waveguide filter which is used in a selective separation of signals in a particular frequency band in the field of a mobile communication, satellite communication, fixed microwave communication and other communication technologies, in particular, to such filter constructed with a coplanar line, and a method of forming same.
Recently, a coplanar waveguide filter constructed with coplanar lines is proposed to be used as a filter which is used in the separation of signals in the transmission and reception of a microwave communication. The concept of a coplanar line will be described with reference to FIG. 1.
In FIG. 1, formed on a dielectric substrate 1 are a ribbon-like center conductor 2 and a first and a second ground conductor 3a and 3b disposed on the opposite sides of the center conductor 2 with an equal spacing therebetween. The three members including the center conductor 2, the first and the second conductor 3a and 3b are formed parallel to and coplanar with each other on the common surface of the dielectric substrate 1. The coplanar line has features that no via-holes are not required in forming an inductive coupler, a miniaturization is possible without changing a characteristic impedance and that a greater freedom of design is available. Denoting the width of the center conductor 2 by w and the spacing between the center conductor 2 and each of the first and the second ground conductor 3a and 3b by s, the coplanar line has a characteristic impedance which is determined by the line width w of the center conductor 2 and the spacing d(w+2s) between the first and the second ground conductor 3a and 3b. 
Referring to FIGS. 2A to 2C, a conventional example of the coplanar wave guide filter will now be described where a first to a fourth resonator 5a to 5d are disposed on a line. Each resonator comprises a center conductor 2 having an electrical length equivalent to one-quarter wavelength and a first and a second ground conductor 3a and 3b disposed on the opposite sides of and parallel to the center conductor 2 and spaced therefrom by a spacing s, which are formed on the common surface of a dielectric substrate 1.
A first input/output terminal section 4a of a coplanar waveguide to which a signal is input is capacitively coupled to the first resonator 5a. In the example shown, one end of a center conductor line 24a of the first input/output terminal section 4a and one end of a center conductor line 2R1 of the first resonator 5a are disposed in mating relationship with each other in the manner of comb teeth and spaced by a gap g1 in order to strengthen the capacitive coupling, thus forming a first capacitive coupler 6a. The other end of the center conductor line 2R1 and one end of a center conductor line 2R2 of a second resonator 5b are connected together by shorting line conductors 7a1 and 7a2 which are connected to the first and the second ground conductor 3a and 3b, respectively, thus forming a first inductive coupler 8a between the first and the second resonator 5a and 5b. 
Cuts 20 are formed into the first and the second ground conductor 3a and 3b on each side of the shorting line conductors 7a1 and 7a2, whereby the shorting line conductors 7a are apparently extended, increasing the degree of coupling of the first inductive coupler 8a. A gap g2 is provided between the other end of the center conductor line 2R2 of the second resonator 5b and one end of a center conductor line 2R3 of a third resonator 5c, whereby the second and the third resonator 5b and 5c are coupled together by a second capacitive coupler 6b. 
The other end of the center conductor line 2R3 and one end of a center conductor line 2R4 of a fourth resonator 5d are connected together by shorting line conductors 7b1 and 7b2 and connected to ground connectors 3a and 3b, whereby the third and the fourth resonator 3c and 5d are coupled together by a second inductive coupler 8b. In the second inductive coupler 8b, also cuts 20 are formed into the ground conductors 3a and 3b. 
The fourth resonator 5d and a second input/output terminal section 4b are capacitively coupled. Specifically, the other end of the center conductor line 2R4 and a center conductor line 24a of the second input/output terminal section 4b are formed in the configuration of meshing comb teeth and disposed in opposing relationship and spaced apart by a gap g3, thus forming a third capacitive coupler 6c which provides a strong coupling therebetween.
As mentioned above, the characteristic impedance of the coplanar line is determined by the width w of the center conductor line and the ground conductor spacing d(w+2s) between the first and the second ground conductor 3a and 3b. However, the resonators 5a, 5b, 5c and 5d which form together a conventional waveguide filter has a characteristic impedance of 50Ω which is the same as the characteristic impedance of various devices connected to the input/output terminal section 4 for the ease of design. (See, for example, H. Suzuki, Z. Ma, Y. Kobayashi, K. Satoh, S. Narashima and T. Nojima: “A low-loss 5 GHz bandpass filter using HTS quarter-wavelength coplanar waveguide resonators”, IEICE Trans. Electron., vol. E-85-C, No. 3, pp 714-719, March 2002.)
Accordingly, in the practice of forming the coplanar waveguide filter, a pattern such as shown in FIG. 1A is formed by an etching of conductor films on a dielectric substrate by designing a filter which satisfies an intended filter response with a characteristic impedance of 50Ω while choosing a ground conductor spacing d1 and a center conductor line width w1 of an input/output terminal section which are equal to a ground conductor spacing d2 and a center conductor line width w2 of a resonator, respectively. Power is fed to the resulting coplanar waveguide filter and a maximum input power is determined so that a power loss which occurs is equal to or less than a given value or if a superconducting material is used to form a conductor film which is etched, a maximum power input is determined so as to avoid a loss of the superconducting state. In other words, a maximum input power level could not have been determined until after a filter has been formed.
FIG. 3 graphically shows a current density distribution of a conventional coplanar waveguide filter. In FIG. 3, the X-axis represents the direction of length of the coplanar line while Y-axis represents a direction which is orthogonal thereto, and a current density at a given coordinate is indicated along the ordinate. It will be seen from FIG. 3 that the current density is at its maximum on the edge line 9 (indicated in thick lines) of the first and the second inductive coupler 8a and 8b, as will be further described later, and this has been an essential factor which causes an increased power loss.
The current density assumes a maximum value of about 2200 A/m at the first inductive coupler 8a which is located at a distance of about 8.5 mm from the input of the coplanar line and also at the second inductive coupler 8b which is located at a distance of about 20 mm from the input. FIG. 4 graphically shows a current density distribution of the first inductive coupler 8a to an enlarged scale. The position along the X-axis shown in FIG. 4 represents a length as referenced to a signal input end of the first input/output terminal section 4a shown in FIG. 2, and a position corresponding to 8.892 mm is indicated in FIG. 2 by a line IV-IV. Specifically, an X-axis position which steps back by 0.014 mm toward the input from the lateral edge of the shorting line conductor 7a1 which is located toward the second resonator 5b represents 8.892 mm position shown in FIG. 4. FIG. 4 shows a current density distribution in the range of 0.1 mm from this position toward the output. It will be seen that the current density is particularly high at two locations including a corner α where the shorting line conductor 7a1 contacts the first ground conductor 3a and another corner β where the shorting line conductor 7a1 contacts the center conductor line 2R2 and that the current is concentrated at a corner γ located on the opposite side from the corner α of the rectangular cut 20 into the first ground conductors 3a which is provided for the purpose of increasing the degree of coupling of the inductive coupler 8. Such peaks of the current concentration also occur at respective corners which are located in line symmetry with respect to the centerline which is drawn through the center of the width of the shorting line conductor 7a1 from the corners α, β and γ. A particularly high current concentration peak occurs at three corners α, β and γ. It should be understood that the same tendency prevails on the side of the second ground conductor 3b, producing a current concentration at each corner between the shorting line conductor 7a2 and the center conductor line 2R2 and the second ground conductor 3b. 
In a conventional filter, an approach to increase the degree of coupling of the inductive coupler has been to reduce the width of the shorting line conductors 7a1 and 7a2 or to increase the substantial length of the shorting line conductors by providing cuts 20 into the ground conductors 3. As a result of such approach, the current concentration occurs at corners of the shorting line conductor which forms the inductive coupler and there arises a problem in a filter in which the conductive films on the dielectric substrate are formed of a superconducting material that the superconducting state is destructed by the occurrence of a current concentration which exceeds a critical current density if the resonator were refrigerated below a critical temperature.
There also arises a problem that the configurational construction of the shorting conductors 7a1, 7a2, 7b1 and 7b2 becomes finer or complicated, presenting a difficulty in securing the accuracy of design.
The present invention has been made in consideration of these aspects, and has for its object the provision of a coplanar waveguide filter which reduces a maximum current density in a resonator and avoids an increase in the power loss with a construction which assures that the accuracy of design can be maintained and which prevents a superconducting state from being destructed if component conductor films were formed of a superconducting material.
It is also to be understood that in a conventional method of forming, the power of a filter input signal is determined after a coplanar waveguide filter has been formed, and it has been difficult to manufacture a filter having a desired response with respect to a predetermined power of the input signal.